I can relate to your question. I had asked a similar question in this Post.
Just to clarify your question... are you talking hole or bulkhead size?
If you are talking bulkhead size:
Using this calculator, the difference is 1300 gph vs 2200 gph. obviously your overflow must be larger to feed the large bulkhead. The calculator provides the numbers of 20 vs 33 of linear inches minimum. The calculator is not clear what this linear inches really is. I have assumed this is total overflow length. My overflows have teeth which means my 32 inches of overflow must have atleast 3/16" of water above the lowest part of the teeth to feed my 4 each 1" bulkheads.
If you are talking hole in glass size:
That is harder. My 1" threaded bulkhead required a 2" hole while my 1" slip bulkhead was closer to 1.5" hole. I would put in thread bulkheads if you think you might want to change things later.
It is better to have an oversized oversized than undersize, that's for sure.
I just want to add that it is best to get the bulkhead before drilling holes. Every brand of bulkhead has different outside diameter. Some one incher needs a 2" hole, while some needs 1 5/8. All different.
